There are a host of different alarm systems presently being marketed, however, the more common alarm systems use some sort of motion detection. One of the most common motion detectors has a dual detector design typically with a microwave detector sub-system as well as a passive infrared motion detector sub-system. These dual type alarms are believed to be more reliable and reduce the probability of false alarms, as an alarm is produced only when both of the sub-systems detect motion.
Some systems include an arrangement for determining whether both detectors are working, as an alarm output is only generated when both detectors are functioning and have detected motion. This monitoring of the detectors is often described as trouble analysis. If there is a trouble condition with respect to one of the detectors, then the dual detector sensor can convert itself to the single operating technology. When there is an output from the trouble monitoring of the system, typically the operator or the customer is alerted to this fact and investigates the system. Unfortunately, detectors of this design produce more trouble output signals than are indicative of an actual trouble condition. This is very undesirable and represents a serious impairment.
In an effort to overcome this situation, some prior art detector systems count the number of detections generated by each of the two detecting sub-systems and then uses the resulting counts to infer therefrom the state of the device. This technique attempts to extract trouble information from sub-systems designed to detect motion. As a result, the sensitivity of the supervision mechanism is totally dependent upon the individual detection sub-system's sensitivity, which is typically high to ensure overlapping coverage patterns for both technologies. Furthermore, the technique falsely assumes that both detection patterns precisely overlap, so that anything which trips one technology is expected to trip the other. Since the product designer has very little control over the radiated field pattern of the microwave sensor, there will always be some lack of overlap in detection coverage. Also, since the transducers physically detect motion differently and due to the randomness, inconsistency and unpredictable nature of both human motion and noise signals (both environmental and internal), it is unreasonable to always expect simultaneous verification of a valid event. This counting approach is prone to false trouble indications which are more of a nuisance to the installers and customers than a benefit.
A different approach involves active supervision. Active supervision includes devices which simulate activity in the supervised area which activity is sensed by the transducers. At predefined intervals, these devices are activated and the resulting alarm outputs are registered in the device. Therefore, the device is tested to determine whether both transducers are functioning and a trouble signal is produced when these tests conditions do not result in both transducers detecting the motion. This is a fairly complicated and expensive arrangement and is not particularly practical.
There remains a need to provide a simple arrangement for analyzing of signals from motion detectors for both alarm and trouble conditions.